This report compares the relative levels of messenger RNA species coding for plasma proteins in rat visceral yolk sac and fetal liver from 12.5 days to 21.5 days gestation. Transthyretin, retinol-binding protein, transferrin and alpha 1-fetoprotein mRNAs were detected in both tissues, although relative levels were much higher in the yolk sac compared to fetal liver, in early gestation. Messenger RNA coding for the positive acute phase proteins thiostatin, fibrinogen, alpha 2-macroglobulin and alpha 1-antitrypsin were detected at a low but significant level in yolk sac, while the levels in fetal liver steadily increased from 16.5 days gestation and, with the exception of alpha 1-antitrypsin, reached levels higher than those found in adult liver just prior to birth. Albumin, inter-alpha 1-trypsin inhibitor, alpha 1-acid glycoprotein, haptoglobin, vitamin D-binding protein and ceruloplasmin messenger RNA levels were either very low or undetectable in yolk sac and fetal liver. Secretion of proteins by yolk sac endoderm occurred largely across the basolateral surface, i.e. towards the fetal compartment. These data support the hypothesis that one function of the yolk sac in the rat is the synthesis and secretion of a select group of plasma proteins to maintain homeostasis in the fetal compartment in the period before the fetal liver has matured sufficiently to carry out this function.